PERFORATION
PERFORATION
Perforation is the process that
creates a direct link between the
wellbore and a producing
formation by puncturing holes
through the casing and the
cement sheath that surrounds it.
Perforation involves specialized equipment that
creates tiny holes through the casing, the
surrounding cement, and any other barrier
between the formation and the open well.
A number of types of completion techniques have
been developed, and the method selected for a
given application depends on the characteristics
and location of the formation.
Perforating creates a path from the confined environment built
by drilling & completion engineers to the expansive formations
imagined by reservoir engineers.
PERFORATION Introduction
Cased-hole completions require casing to be run into the
reservoir. In order to achieve production, the casing and
cement are perforated to allow the hydrocarbons to enter
the wellstream.
PERFORATION
PERFORATION Cased Hole Completion
• Wells are cased off to allow a safe and controlled
well/reservoir interface.
• To provide the interface the well must be perforated.
Oil or gas flow
Perforations Casing & Cement Formation
PERFORATION
• Perforations are made by
firing shaped charges
downhole through the
casing & cement into the
formation, using a
Perforating Gun.
Shaped
Charge
PERFORATION Where to perforate
While the perforation locations have been
previously defined by drilling logs, those intervals
cannot be easily located through the casing and
cement.
To overcome this challenge, a gamma ray-collar
correlation log is typically implemented to correlate
with the initial log run on the well and define the
locations where perforation is required.
PERFORATION
PERFORATION
A Petroleum
Technologist’s job:
Ensure that perforation is
done at the correct depth!
The natural gamma ray (GR) module measures the amount of gamma
radiations occurring naturally within the formations crossed by a borehole. A
scintillation Sodium Iodide crystal is used to detect the gamma rays. Gamma
rays are produced mainly by isotopes of potassium, thorium, uranium and
their decay products. The gamma ray log is widely used in the mining and
groundwater industry for the identification of lithology, correlation between
boreholes and clay content analyses.
The casing collar locator (CCL) is a magnetic device sensitive to changes of
metal thickness at casing or tubing collar. It is always run in steel cased
boreholes to detect the position of casing collars. It is mainly used for depth
control and depth correlation.
PERFORATION Purposes of Perforation
Creates a channel between the pay
zone and the wellbore.
Causes oil and gas to flow to the
wellbore easily.
Future stimulation.
example: Hydraulic fracturing
PERFORATION Perforation geometry
PERFORATION Perforating Guns
A device used to perforate oil and gas wells
in preparation for production.
It has different shapes of explosive charges
Different range of sizes and configurations.
The diameter of the gun used is determined
by the presence of wellbore restrictions or
limitations imposed by the surface
equipment.
PERFORATION Types of Perforating Guns
Casing gun
Expendable gun
Retrievable gun
High-shot density gun
PERFORATION Casing gun
Create holes in a casing string .
Casing guns are typically 3- to 5-in. in
diameter and carry up to four
perforating charges per foot
Allow perforation through production
casing using larger diameter gun
assembly.
PERFORATION Expendable gun
A perforating gun assembly that
disintegrates upon firing, creating finer
debris.
It is used where wellbore restrictions
allow only limited access, as in through-
tubing applications.
It is relatively light and simple in design
with phased expendable guns.
PERFORATION Expendable gun
PERFORATION Retrievable gun
It is retrievable from the wellbore after
firing.
It generate minimal debris
Minimum distortion of the gun body to
help ensure easy retrieval.
PERFORATION High-shot density gun
A perforating gun which has more than
four shots per foot.
Improves the phasing, or distribution of
perforations, around the wellbore.
Enables improved distribution of the
perforations around the circumference
of the casing or liner.
PERFORATION High-shot density gun
PERFORATION Perforation methods
Bullet Perforation
Jet Perforation
Hydraulic “Sand-jet” perforation
Perforation using laser technology
Horizontal oriented perforating system
– HOPS
PERFORATION Perforation methods
You perforate with wireline, slickline or coiled tubing.
Wireline is quicker, but there is a weight limit on the
length of guns you can run - Strip Guns or Hollow
Carriers.
Strip Gun
Hollow Carrier
If a lubricator is used, the guns can be lowered
into a live well, through coiled tubing.
PERFORATION Perforation methods
High Shot Density
PERFORATION Perforation methods
Tubing conveyed perforating guns (TCP) can only be used with
Hollow Carrier or Casing guns (i.e. guns where the charges are
inside a steel pipe).
Strip guns can’t be used as they might buckle under the weight
of the tubing, if they get stuck on something as they’re run into
the hole.
PERFORATION
In addition to these 2 types of gun systems, the guns
can be fired in over-balanced or under-balanced
conditions (i.e. the bottom hole pressure in the well at
the perforations is greater than or less than the reservoir
pressure, respectively).
i.e. Overbalance, Pwf > Pres
Underbalance, Pwf < Pres
The negative differential pressure obtained during
underbalanced drilling between the reservoir and the wellbore
encourages production of formation fluids and gases. In contrast
to conventional drilling, flow from the reservoir is driven into the
wellbore during underbalanced drilling, rather than away from it.
PERFORATION Underbalanced Drilling
Underbalance Though not as common as
Pwf < Pres overbalanced drilling,
underbalanced drilling is
achieved when the pressure
exerted on the well is less than or
equal to that of the reservoir.
Performed with a light-weight
drilling mud that applies less
pressure than formation pressure,
underbalanced drilling prevents
formation damage that can occur
during conventional, or
overbalanced drilling processes.
PERFORATION Perforation methods
Tubing
Electric Conveyed
Line with Perforating
Hollow with Hollow
Carriers Carriers
Electric Line
with Strip Guns
PERFORATION Bullet Perforation
Bullets lose velocity when gun clearance >0.5 in.
Often used in unconsolidated formations
Cheaper to use, rarely been used today
Bullets plug the end of the flow channel
PERFORATION Bullet Perforation
Advantages of Bullet Perforator:
• Bullets cost les than jet perforator.
• Bullets cause formation fracturing that may
increase permeability in thick or tight formations
and improve the well performance.
• It is possible to obtain a round burrless hole with
bullets which is desirable to treat a perforated
section selectively by plugging some of the
perforations temporarily with small balls.
Disadvantages:
• Bullets cause fracturing of cement and formations.
• This is disadvantageous when perforating thin
zones near water or gas-bearing formations.
• Fracturing of the cement may cause channelling of
unwanted fluids in to the perforations.
• Bullets may not be used in high temperature wells.
• This pistol powder used in bullet perforators is
limited to about 250oF.
• Bullets usually do not attain as deep a penetration
as shape charges
Productivity Ratio vs Penetration
PERFORATION for Various Shot Densities
Shot Densities
1.4
re = 660 ft
Shots per foot Improvements
1.2 rw = 0.25 ft
also come from
rp = 0.8 in (entrance hole) 4
bigger holes,
1.0 2
optimal phasing
Production Ratio
0.8 1 and increased
drawdown and
0.6 clean-up period
during and after
0.4
perforation
Adopted from Harris
0.2 Productivity ratio is
ratio PI for perforated
0 completion to PI for
0 2 4 6 8 10 12 14 open hole completion
Perforation Penetration (inches)
PERFORATION Jet Perforation
It uses a shaped charges contained in a
perforating gun assembly.
Gun assembly is placed in wireline, or
coiled tubing, depending on the
application and the wellbore conditions.
High-pressure jet will penetrates the
casing or liner to shoot into the
reservoir formation to form channels.
PERFORATION Jet Perforation
Components of a shaped charge
• Liner – serves to protect the main explosive
charge & which ultimately produces the jet
action.
• Primer charge – to ignite the detonator
Mechanism of Jet-slug formation
• The jet that forms and also a larger slower
moving secondary slug of metal, which is
sometimes called a carrot.
Components of Jet Perforator
• outer case
• Explosive
• Prima Cord
• metallic liner.
Outer Case
• The outer case is manufactured from a variety
of materials depending on the desired
mechanical characteristics.
• Steel and zinc are the most common
materials, however aluminium, ceramic and
glass are also used to form the case.
• Regardless of the material used to form the
case, tight tolerance must be met to ensure
correct operation of the shape charged.
Explosive:
• The selection of explosive material is based on the well
temperature and anticipated exposure time at that
temperature.
• RDX, HMX (high melting explosive), PYX
(Pikrylaminodinitropyridin) and TACOT (thermally stable
high explosive) are explosive used in oil well shaped
charges.
• The temperature versus exposure time curve for each of
the exposure commonly used in shaped charges are the
basis for the selection.
• Most of the wells are in the range of the RDX temperature
range and therefore are perforated with shaped shape
charged lodged with RDX.
Liner
• The liner is the most important element of the charge.
• The final performance of the shape charge is largely
dictated by the liner.
• A blend of various metallic powders are compressed to
form the liner to avoid slug obstruction in perforation.
• This allows the slug to disintegrate the explosion.
• Liners are formed from mixtures of copper, lead, zinc, tin or
tungsten.
• The density distribution and dimension of the liner is highly
controlled to maintain consistent, high performance.
Operation of Jet Perforator
• When the shape charged is installed in a gun the
detonating cord runs along the gun’s length and
contacts each charge at the primer region.
• After the gun is positioned correctly in the well the
detonating cord is initiated by either applying current
to an electric detonating cap or striking a nonelectric
detonating cap with a firing pin.
• The wave front from the detonating cord travels at
about 7000 m/s and reaches pressures around 15 to
20 Gpa.
• This wave front initiates the fine grained primer
explosive that fills the short tunnel connecting the
detonating cord with the bulk of the explosive.
• The detonation accelerates as it reaches the liner
apex. At this point the front advances spherically
at 8000 m/s and develops pressure of 30 Gpa.
• At this pressure the charge case and line provide
very little mechanical resistance.
• The case is propelled outward and the outer shell
of liner is collapsed to form slower moving metal
stream with a velocity between 1500 and 3000 ft./
sec.
Formation of jet carrot
Metal in liners becomes fluid at very high Pressure
High speed jet with extremely fine metal
Detonation of Jet Perforator
Liner and outer part of cone separate
High speed jet of extremely
PERFORATION Jet Perforation
PERFORATION Jet Penetrating Mechanism
SHAPED
SLUG
CHARGE V1
P1 0.5
JET
Pressure on Target : P1 = 5 x 106 PSI
Velocity of forward jet : V1 = 20,000 FT./SEC.
Low Temperature
Formation Material simply flows plastically and radially
away from the point of impingement of the jet
SHAPE CHARGED OR
JET PERFORATORS
• An electrically fired detonator starts a chain reaction which
successively, detonates the prima-cord of the jet perforator.
• The high velocity booster gets charged and finally the main
explosive.
• High velocity generated by explosive causes the metal in the
charge lines to flow separating the inner and outer layer of
the liner.
• Continues pressure build up on the liner causes a needle like
high speed jet of fine particles to evolve from the gun at a
speed of 20,000 ft/sec. at its tip with a pressure of 5 million
psi.
PERFORATION Jet Perforation
PERFORATION Jet Perforation
Hydraulic “Sand-jet” Perforation
It uses high-pressure jetting of sand-
laden fluid through an orifice
Short penetration range
PERFORATION Perforating with Laser
Conventional explosive charge
perforation method reduces the rock
permeability
Laser perforation increase the rock
permeability, hence, increase the oil or
gas production rate of a well.
PERFORATION Perforating with Laser
PERFORATION
Horizontal oriented perforating system – HOPS
Performs oriented perforating
Incorporating a series of swiveled gun
connectors containing internal, explosive
transfer devices and an internally contained
gun weighting system.
This system incorporates a series of short gun
sections that rotate to the desired
orientation, providing the optimum shot
pattern and maximizing reservoir production.
PERFORATION
Horizontal oriented perforating system – HOPS
PERFORATION Problem with Perforation
The disadvantage is that perforating can lead to "skin damage",
where debris from the perforations can hinder productivity of the
well.
In order to mitigate this, perforating is commonly done
underbalanced (lower pressure in the well bore than in the
formation) as the higher formation pressure will cause a surge of
fluids into the well at the point of perforating, hopefully carrying
the debris with it.
Other methods of stimulation such as acidizing and proppant
fracturing are often required to overcome this damage and bring
the well up to its full potential.
PERFORATION Problem with Perforation
Adjusting Perforation Balance
The pressure differential between a well bore and the
reservoir prior to perforation can be described as under-
balanced, balanced or over-balanced.
A desirable under-balance condition exists when hydrostatic
pressure inside the well casing is less than pressure in the
formation.
PERFORATION Problem with Perforation
Unfavorable Over-balance
It is commonly accepted that perforations conducted in over-
balanced conditions can result in damage to the rock matrix.
SPE Paper 63108I (“Optimum Under-balance for the Removal of
Perforation Damage,” I.C.Walton, 2000) reported that such a
damage zone typically extends about 1 centimeter into the rock,
and results in 20 percent or more of permeability reduction.
PERFORATION Problem with Perforation
Perforation causes formation damage, and reduces
permeability.
The main problem is the plugging after Perforation, that are
caused by:
– Charge liner residue slugs
– Mud solids & crushed/compacted rock particles
The following slide illustrates how a deep penetrating shaped
charge penetrates the wellbore components.
PERFORATION
Explosive events, such as perforating, create very high pressures, necessary to
penetrate the well casing, that also crush and damage the rock.
PERFORATION Problem with Perforation
On a microscopic basis, the very high load and load rate
(millions of psi in microseconds at the jet center) shatter
the individual formation sand grains, as well as reduce the
cement particles to a very fine powder.
This diminished particle size creates a surrounding filter
cake, reducing permeability and inhibiting effective
subsequent fluid injection.
PERFORATION Problem with Perforation
Lower permeability rocks tend to exhibit a greater
percentage of permeability reduction under conditions of
over-balance.
Perforating shock waves and high impact
pressure shatter rock grains, break down inter-granular
mineral cementation and de-bond clay particles, resulting in
a low-permeability crushed zone in the formation around
perforation tunnels.
Damage to the rock matrix is caused by crushing of sand
grains as the jet enters the rock, as illustrated in the next
slide.
PERFORATION Problem with Perforation
Perforation shock waves and high impact pressure shatter rock grains that break
down inter-granular mineral cementation and de-bond clay particles. This
creates a low permeability zone in the formation around perforation tunnels
to reduce flow potential.
PERFORATION Problem with Perforation
In contrast, under-balance perforation facilitates flow from
the formation into the tunnel (see slide below).
This flow aids in the removal of perforating debris while
minimizing or eliminating crushed-zone damage in and
around the perforation tunnel.
PERFORATION Problem with Perforation
Under-balanced perforation improves flow channels by effectively
removing the crushed zone through an instantaneous surge of fluids from
the reservoir into the wellbore when the jet penetrates the rock.
PERFORATION Perforation Tunnel Cleaning
Perforating Debris
Crushed Zone
Casing Invaded Zone Undamaged Formation
Cement
Perforating
underbalance
Casing Invaded Zone Undamaged Formation
improves Cement
perforation clean-up
PERFORATION Problem with Perforation
The benefit of this strategy is greater deal of control of the well.
Casing the bottom of the hole allows the well to be completed
without having to worry about reservoir fluids.
It also allows precise selection of where in the formation
production will be and to be able to seal off perforations, which
are no longer useful or counterproductive, through cementing
or straddling.
PERFORATION Summary
Different perforation guns have specific application.
Example: casing gun, retrievable gun, high-shot
density gun, & expandable gun.
Perforation methods include: Bullet, Jet, Hydraulic,
& Laser.
Problems exist in perforation.
Casing and perforating as a method of completion is
common place nowadays, though in some
unconsolidated formations, prone to production of
sand open hole completions, using only
sandscreens, may be the preferred choice.
PERFORATION
Commonly, perforation guns are run on E-line as it is
traditional to use electrical signals from the surface to fire
the guns.
In more highly deviated wells, coiled tubing may be used.
Newer technologies allow the guns to be run on slickline. No
communication with the surface is possible with slickline.
Instead, a mechanism on the gun arms the charges upon
reaching a certain temperature and pressure. A timer will
then fire them following a set interval.
PERFORATION
